The present invention relates to a capacitance element using a capacitance insulating film made of a dielectric material with a high dielectric constant or of a ferroelectric material and to a manufacturing method therefor.
As higher-speed and lower-power microcomputers have been implemented in recent years, electronic devices to be used as consumer products have remarkably increased in performance, while semiconductor elements composing a semiconductor device used therein have been rapidly scaled down. Under such circumstances, undesired radiation which is electromagnetic noise generated from the electronic devices has presented a serious problem. As a measure to suppress the undesired radiation, attention has been focused on the technique of embedding, in a semiconductor integrated circuit or the like, a capacitance element with large capacitance using a capacitance insulating film made of a dielectric material with a high dielectric constant (hereinafter simply referred to as a high-dielectric-constant material). As higher integration has been achieved in a dynamic RAM, on the other hand, extensive research has been conducted on the technique of using a high-dielectric-constant film as a replacement for a silicon oxide film or silicon nitride film that has been used previously. Additionally, vigorous research and development has been directed toward a ferroelectric film having the property of spontaneous polarization to implement an industrially usable non-volatile RAM capable of operating at low voltage and performing high-speed writing and reading operations.
To implement a semiconductor device having the performance described above, it is important to devise a capacitance element having such a structure as to allow higher integration without degrading the properties of the capacitance element and a manufacturing method therefor.
Referring to the drawings, a conventional capacitance element and a manufacturing method therefor will be described. FIG. 9 is a cross-sectional view of a principal portion of the conventional capacitance element, in which are shown: a substrate 21 such as a silicon substrate with an integrated circuit formed therein; a lower electrode 22 of the capacitance element which is composed of a platinum film or the like; a capacitance insulating film 23 of the capacitance element which is composed of a thin ferroelectric film; and an upper electrode 24 of the capacitance element which is composed of a platinum film or the like. The upper and lower electrode 24 and 22 and the capacitance insulating film 23 constitute the capacitance element. There are also shown: an aperture 25 formed in the capacitance insulating film 24; an interlayer insulating film 26 covering the capacitance element; a first contact hole 27 extending through the interlayer insulating film 26 to reach the lower electrode 22; a second contact hole 28 extending through the interlayer insulating film 26 to reach the upper electrode 24; a first electrode wire 29 to be connected to the lower electrode 22; and a second electrode wire 30 to be connected to the upper electrode 24.
The recent trend has been to compose each of the electrode wires 29 and 30 of a multilayer film such as a two-layer film consisting of an upper-layer aluminum-alloy film containing aluminum as a main component and a lower-layer titanium film or a three-layer film consisting of an upper-layer aluminum-alloy film containing aluminum as a main component, an interlayer titanium nitride film, and a lower-layer titanium film. In the case of embedding such a capacitance element in an integrated circuit, in particular, the first and second electrode wires 29 and 30 are also connected directly to a diffusion region in the integrated circuit, so that the titanium film is normally used to compose the lowermost layer of the multilayer film, thereby lowering contact resistance between the diffusion region and the aluminum alloy film.
Next, a description will be given to the manufacturing method for the conventional capacitance element. FIGS. 10(a) to 10(e) are cross-sectional views illustrating the process of manufacturing the conventional capacitance element.
First, in the step shown in FIG. 10(a), a first platinum film 22a, a ferroelectric film 23a, and a second platinum film 24a are formed sequentially on the substrate 21. Next, in the step shown in FIG. 10(b), the second platinum film 24a is patterned by using a photoresist mask to form the upper electrode 24. Next, in the step shown in FIG. 10(c), the dielectric film 23a is patterned by using a photoresist mask covering a region including the upper electrode 24 to form the capacitance insulating film 23 having the aperture 25. Furthermore, the first platinum film 22a is etched selectively by using a photoresist mask covering the upper electrode 24, the capacitance insulating film 23, and the aperture 25 to form the lower electrode 22.
Next, in the step shown in FIG. 10(d), the interlayer insulating film 26 is formed on the substrate, followed by the first contact hole 27 formed to extend through the interlayer insulating film 26 to reach the lower electrode 22 and the second contact hole 28 formed to extend through the interlayer insulating film 26 to reach the upper electrode 25.
Next, in the step shown in FIG. 10(e), the titanium film and the aluminum alloy film are deposited over the entire surface of the substrate. The titanium film and the aluminum alloy film are then patterned by using a photoresist mask covering the contact holes 27 and 28 and their surroundings to form the first electrode wire 29 to be connected to the lower electrode 22 and the second electrode wire 30 to be connected to the upper electrode 24.
Although each of the first and second electrode wires 29 and 30 is shown as a single-layer film in FIG. 10(e) for the sake of simplicity, it is typically composed of a multilayer film such as the two-layer film consisting of the aluminum alloy film and the titanium film or the three-layer film consisting of the aluminum alloy film, the titanium nitride film, and the titanium film as described above.
In the conventional capacitance element, excellent adhesion is required between the second electrode wire 30 and the upper electrode 24. Moreover, since the capacitance insulating film 23 is typically composed of a ferroelectric material containing a metal oxide as a main component, the platinum film is used to compose each of the upper and lower electrodes 24 and 22 as a material which is unreactive to the metal oxide and capable of withstanding high temperature during thermal treatment. Furthermore, the titanium layer is interposed between the aluminum layer and the platinum layer to compose each of the electrode wires 29 and 30 due to poor adhesion between the aluminum layer and the platinum layer, thereby solidifying the connection between the electrode wires and the electrodes of the capacitance element.
To improve the performance of the capacitance element, thermal treatment is indispensably performed after the formation of the electrode wires 29 and 30 in the manufacturing process. After the heat treatment was performed with respect to the electrode wires 29 and 30, however, the phenomenon was observed in which the performance of the ferroelectric film composing the capacitance insulating film 23 was degraded.
The cause of the degraded performance was tracked down and presumed as follows. The platinum film composing each of the upper and lower electrodes 24 and 22 of the capacitance element has a columnar crystal structure since it is normally formed by sputtering. During the thermal treatment performed with respect to the electrode wires 29 and 30, titanium composing the lower layer of the second electrode wire 30 diffuses into the capacitance insulating film 23 through the grain boundary of the columnar crystal in the platinum film composing the upper electrode 24 to react with the ferroelectric film composing the capacitance insulating film 23, which is the presumed cause of the degraded performance.
The foregoing problem may occur not only in the case where each of the electrodes of the capacitance element is composed of the platinum film but also in the case where it is composed of iridium, palladium, ruthenium, or the like. Even when the lower electrode is composed of a polysilicon film as in a storage node of a memory cell transistor in a DRAM, a similar problem occurs provided that the upper electrode is composed of platinum or the like.